Study: What Environmental Impact Do Scrubbers Have on the Baltic Sea?

Scrubbers, also known as gas washers or exhaust gas cleaning systems, are meant to clean ship emissions using seawater and reduce the acid-forming sulfur emissions into the air. However, they simultaneously create a new source of emissions, according to researchers from Chalmers University of Technology in Gothenburg, Sweden. The wash water, which is acidified and contains various heavy metals and other toxins, is discharged into the sea.

For a long time, there was a lack of knowledge about the effects of exhaust gas cleaning on the wash water that is dumped into the sea. Chalmers University of Technology continued an investigation commissioned by the Swedish Transport Administration and the Swedish Agency for Marine and Water Management, as assigned by the government.

Environmental Impact

The investigation aimed to find out the environmental impact of scrubbers in the Baltic Sea compared to other environmental pollutants. The result: According to the study, the wash water from scrubbers accounts for up to nine percent of the total amount of certain carcinogenic and environmentally harmful substances emitted into the Baltic Sea—significantly more than was previously known.

The researchers referred to so-called polycyclic aromatic hydrocarbons, or PAHs for short. Additionally, the number of ships with scrubbers has more than tripled since the study was conducted.

"We have been warning for many years that exhaust gas cleaning is a technology that stands for disproportionately high discharges of dangerous and acid-forming substances into the marine environment. Nevertheless, a sharp increase in the number of installed scrubbers has been observed, driven by economic advantages for shipping companies," said Ida-Maja Hassellöv, assistant professor and researcher in the Department of Marine Sciences at Chalmers, adding, "Therefore, it is of great importance that authorities and decision-makers wake up soon and pass regulations to reduce the emissions and impacts of shipping on the marine environment."

Dirty Wash Water

The study, published in the Marine Pollution Bulletin, found that ships cleaning their exhaust gases discharged more than 200 million cubic meters of environmentally harmful wash water into the Baltic Sea within a year. Moreover, ships with copper-based antifouling paint contribute one-third of the total copper amount entering the Baltic Sea, according to another finding of the study.

Copper in hull coatings has long been known as an environmental problem because heavy metals cannot break down in the environment, leading to high levels in water, sediments, and soil. However, the exact contribution of boat and ship traffic to the total copper input had been unknown until now.

The carcinogenic PAHs are also persistent and can spread widely in the environment before they degrade. According to Erik Ytreberg, a lecturer and marine scientist at Chalmers and one of the study’s main authors, the findings show the significant extent of the shipping industry's contribution to hazardous substances in the Baltic Sea.

"PAH hydrocarbons are extremely toxic to humans and aquatic organisms, among other things because they are carcinogenic. However, it is important to note that the data from the study were collected in 2018. At that time, about 180 ships in the Baltic Sea used scrubbers. Since then, the number of such ships has increased significantly—by 2021, there were already 600 ships in the Baltic Sea operating with scrubbers," explained Ytreberg.

Emissions from shipping were calculated using a model that allows the real-time positioning of a ship to be determined. Various databases were linked with information on whether a ship is equipped with scrubbers. The researchers were thus able to model the quantities and volumes of hazardous substances discharged into the Baltic Sea by the ships.

Time to Act

As a result of these findings, the Swedish Agency for Marine and Water Management and the Swedish Transport Administration have proposed a ban on discharging so-called scrubber water into the inner waters of the Swedish Baltic coast. According to Ytreberg, Sweden would then be the first Scandinavian country to have such a ban.

Among the Baltic Sea countries, only Germany has had such legislation for some time; several European countries have regulated the discharge of scrubber water in ports. However, the Chalmers scientists are hoping for broader regulations in the future.

"The proposal that Sweden wants to ban the dumping of wash water in its inner waters is, of course, good. But it will only regulate one to two percent of the scrubber discharges that exist in the Baltic Sea today," said Ytreberg. "Sweden could also have proposed a ban for its territorial waters, or the coastal sea. That would cover roughly 15–17 percent of scrubber emissions into the Baltic Sea."

However, according to the study’s authors, the greatest impact would come from an internationally agreed ban on discharging wash water by all Baltic sea neighboring countries.